It is well known in the art that a sound produced within a reflective environment may traverse many diverse paths in reaching a receiving transducer. In addition to the direct path sound, delayed reflections from surrounding surfaces, as well as extraneous sounds, reach the transducer. The combination of direct, reflected and extraneous signals result in the degradation of the audio system quality. These effects are particularly noticeable in environments such as classrooms, conference rooms or auditoriums. To maintain good quality, it is a common practice to use microphones in close proximity to the sound source or to use directional microphones. These practices enhance the direct path acoustic signal with respect to noise and reverberation signals.
There are many situations, however, in which the location of the source with respect to the electroacoustic transducer is difficult to control. In conferences involving many people, for example, it is difficult to provide each individual with a separate microphone or to devise a control system for individual microphones. One technique disclosed in U.S. Pat. No. 4,066,842 issued to J. B. Allen, Jan. 3, 1978, utilizes an arrangement for reducing the effects room reverberation and noise pickup in which signals from a pair of omnidirectional microphones are manipulated to develop a single, less reverberant signal. This is accomplished by partitioning each microphone signal into preselected frequency components, cophasing corresponding frequency components, adding the cophased frequency components signals, and attenuating those cophased frequency component signals that are poorly correlated between the microphones.
Another technique disclosed in U.S. Pat. No. 4,131,760 issued to C. Coker et al, Dec. 26, 1978, is operative to determine the phase difference between the direct path signals of two microphones and to phase align the two microphone signals to form a dereverberated signal. The foregoing solutions to the noise and dereverberation problems work as long as the individual sound sources are well separated, but they do not provide appropriate selectivity. Where it is necessary to conference a large number of individuals, e.g., the audience in an auditorium, the foregoing methods do not adequately reduce noise and reverberation since these techniques do not exclude sounds from all but the location of desired sources.
U.S. Pat. No. 4,485,484 issued to J. L. Flanagan on Nov. 27, 1984 and assigned to the same assignee discloses a microphone array arrangement in which signals from a plurality of spaced microphones are processed so that a plurality of well defined beams are directed to a predetermined location. The beams discriminate against sounds from outside a prescribed volume. In this way, noise and reverberation that interfere with sound pickup from the desired source are substantially reduced.
While the signal processing system of U.S. Pat. No. 4,485,484 provides improved sound pickup, the microphone array beams must first be steered to one or more appropriate sources of sound for it to be effective. It is further necessary to be able to redirect the microphone array beam to other sound sources quickly and economically. The arrangement of aforementioned U.S. Pat. No. 4,131,760 may locate a single sound source in a noise free environment but is not adapted to select one sound source where there is noise or several concurrent sound sources. It is an object of the invention to provide an improved sound source detection capable of automatically focusing microphone arrays at one or more selected sound locations.